base/containers/map_util.h

// Copyright 2023 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_CONTAINERS_MAP_UTIL_H_
#define BASE_CONTAINERS_MAP_UTIL_H_

#include <memory>

#include "base/types/to_address.h"

namespace base {

namespace internal {

template <typename Map>
using MappedType = typename Map::mapped_type;

}  // namespace internal

// Returns a pointer to the const value associated with the given key if it
// exists, or null otherwise.
template <typename Map, typename Key = Map::key_type>
constexpr const internal::MappedType<Map>* FindOrNull(const Map& map,
                                                      const Key& key) {
  auto it = map.find(key);
  return it != map.end() ? &it->second : nullptr;
}

// Returns a pointer to the value associated with the given key if it exists, or
// null otherwise.
template <typename Map, typename Key = Map::key_type>
constexpr internal::MappedType<Map>* FindOrNull(Map& map, const Key& key) {
  auto it = map.find(key);
  return it != map.end() ? &it->second : nullptr;
}

// Returns the const pointer value associated with the given key. If none is
// found, null is returned. The function is designed to be used with a map of
// keys to pointers or smart pointers.
//
// This function does not distinguish between a missing key and a key mapped
// to a null value.
template <typename Map,
          typename Key = Map::key_type,
          typename MappedElementType =
              std::pointer_traits<internal::MappedType<Map>>::element_type>
constexpr const MappedElementType* FindPtrOrNull(const Map& map,
                                                 const Key& key) {
  auto it = map.find(key);
  return it != map.end() ? base::to_address(it->second) : nullptr;
}

// Returns the pointer value associated with the given key. If none is found,
// null is returned. The function is designed to be used with a map of keys to
// pointers or smart pointers.
//
// This function does not distinguish between a missing key and a key mapped
// to a null value.
template <typename Map,
          typename Key = Map::key_type,
          typename MappedElementType =
              std::pointer_traits<internal::MappedType<Map>>::element_type>
constexpr MappedElementType* FindPtrOrNull(Map& map, const Key& key) {
  auto it = map.find(key);
  return it != map.end() ? base::to_address(it->second) : nullptr;
}

// Insert or assign into a map type. This has semantics very similar to the
// following statements:
//
//    map[key] = Element(params...)
//    map[key] = element;
//    map[key] = std::move(element);
//    map.insert_or_assign(key, element);
//
// Where key can be any heterogeneous comparable overload of the key_type for
// map. In particular, there is no need to construct a temporary `key` of the
// type stored in the map as long as `key` is comparable with the
// `Map::key_type`.
//
// The `element` is copied or moved into the map, depending on whether it is
// passed by lvalue or rvalue reference.
//
// TODO(crbug.com/376532871): This can be removed once map::operator[] and
// map::insert_or_assign support heterogenous key overloads, in C++26.
template <typename Map,
          typename Key,
          typename MappedElementType =
              std::pointer_traits<internal::MappedType<Map>>::element_type>
Map::iterator InsertOrAssign(Map& map,
                             const Key& key,
                             MappedElementType&& element) {
  auto it = map.lower_bound(key);
  if (it == map.end() || it->first != key) {
    it = map.emplace_hint(it, key, std::forward<MappedElementType>(element));
  } else {
    it->second = std::forward<MappedElementType>(element);
  }
  return it;
}

}  // namespace base

#endif  // BASE_CONTAINERS_MAP_UTIL_H_